In the field of semiconductor fabrication, the use of polysilicon material is diverse. Having a strong resistance for heat, polysilicon materials are commonly used to fabricate gate electrodes for metal-oxide semiconductor transistors. The gate pattern fabricated by polysilicon materials is also used to form self-aligned source/drain regions as polysilicon readily blocks ions from entering the channel region.
However, devices fabricated by polysilicon still have many drawbacks. In contrast to most metal, polysilicon gates are fabricated by semiconductor materials having higher resistance, which causes the polysilicon gate to work under a much lower rate than the metal gates. On the other hand, the conventional polysilicon gate also has faced problems such as unavoidable depletion effect which increases equivalent thickness of the gate dielectric layer, reduces gate capacitance, and worsens a driving force of the devices. Thus, work function metals are developed to replace the conventional polysilicon gate to be the control electrode.
With a trend towards scaling down the MOS size, conventional methods, which are used to achieve optimization, such as reducing thickness of the gate dielectric layer, for example the thickness of silicon dioxide layer, have faced problems such as leakage current due to tunneling effect. In order to keep progression to next generation, high-k materials are used to replace the conventional silicon oxide to be the gate dielectric layer because it decreases physical limit thickness effectively, reduces leakage current, obtains equivalent capacitor in an identical equivalent oxide thickness (EOT), and can be competent to the work function metals.
Materials of the work function metal gates should well operate in both an N-type metal oxide semiconductor (NMOS) device and a P-type metal oxide semiconductor (PMOS) device. Accordingly, compatibility and process control for the metal gate are more complicated, meanwhile thickness and composition controls for materials used in the metal gate method have to be more precise. It is still a challenge to form an optimized work function metal gate to improve the performance of MOS transistors.